P0172

What it means

P0172 means the engine control unit has seen Bank 1 running too rich, so the air-fuel mixture contains more fuel than expected for the air entering the engine. It sets when long-term and short-term fuel trims have pulled fuel back as far as they are allowed and the mixture is still rich. Bank 1 is the side of the engine containing cylinder number one; on a four-cylinder engine it is the only bank.

Symptoms

• Malfunction indicator lamp on
• Higher than normal fuel consumption
• Smell of fuel from the exhaust or black smoke under load
• Rough or unstable idle
• Hesitation or poor performance
• Possible fuel smell in the engine oil

Possible causes

• Faulty or contaminated mass airflow sensor under-reading airflow, or a dirty sensor elementlikely
• Leaking or sticking fuel injector delivering too much fuellikely
• Fuel pressure too high due to a faulty pressure regulator (where fitted) or a restricted fuel return linepossible
• Faulty upstream oxygen or air-fuel ratio sensor giving a false rich signalpossible
• Restricted air intake, such as a blocked air filter or collapsed intake ductpossible
• Purge valve stuck open letting excess fuel vapour into the intake, or a fuel-contaminated crankcase ventilation systempossible
• Engine coolant temperature sensor reading too cold, causing over-fuellingrare

How to diagnose

1. Read the stored codes and freeze frame data, noting engine load, coolant temperature, speed and fuel trim values when the fault set; check for other codes that may point to the cause.
2. Inspect the air filter, intake ducting and airflow sensor for dirt, oil contamination, damage or restriction.
3. Check the wiring and connectors to the airflow sensor, oxygen sensors and coolant temperature sensor for damage, corrosion or poor contact.
4. Check live data: compare short-term and long-term fuel trims at idle and at around 2500 rpm, and check the airflow reading against the expected value for the engine size.
5. Compare the coolant temperature reading with the actual temperature on a cold and a warm engine.
6. Check the upstream oxygen or air-fuel ratio sensor response and whether it agrees with the trims; the procedure depends on the sensor type fitted.
7. Test fuel pressure and check for pressure drop after shutdown against the manufacturer's specification, and look at the fuel return and regulator if fitted.
8. Check the purge valve operation and for fuel in the oil, then test injectors for leakage or imbalance, following the procedure for the engine concerned.

How to fix

• Clean or replace the airflow sensor if it is faulty or contaminated, following the manufacturer's guidance.
• Replace the air filter and repair any restricted intake ducting.
• Replace leaking or faulty injectors, and change the engine oil if it is contaminated with fuel.
• Repair or replace the fuel pressure regulator, or clear a restricted return line, where fitted.
• Replace a faulty oxygen or air-fuel ratio sensor, or the coolant temperature sensor, once testing confirms it.
• Replace a sticking purge valve and repair damaged wiring or connectors.

Can I keep driving?

Nearby codes

• P0171GenericSystem Too Lean Bank 1
• P0173GenericFuel Trim Bank 2